Excimer lasers are used in a number of applications, and one application in which excimer lasers have found particular use is photolithography for chip manufacture. Light from the laser illuminates a photoresist layer spun onto a silicon substrate, and a mask between the laser and the substrate allows some of the laser light to illuminate portions of the photoresist. The photoresist is subsequently developed, and the substrate is etched in areas unprotected by the photoresist.
For some photolithography applications, a laser beam produced within a laser chamber contains light within a range of wavelengths at or near the desired wavelength of 248.3 nm for a KrF excimer laser or 193.3 nm for an ArF excimer laser. The spectral-narrowing assembly removes much of the light at undesired wavelengths and returns the light within a desired narrow range back to the laser chamber. It is very desirable to produce a laser beam having a narrow spectral bandwidth with little variation in spectral bandwidth.
The laser is part of a larger piece of equipment such as a stepper or a scanner that processes the wafers. A typical stepper has a carousel that contains a number of wafers to be etched and otherwise processed to form a finished product such as a computer chip. The stepper removes one of the wafers coated with photoresist from the carousel and positions the wafer in the path of the laser light. The stepper also places the appropriate mask between the coated wafer and the laser light. The stepper assures that all components are properly aligned, and the stepper then instructs the laser to fire and expose a portion of the photoresist on the wafer to a series of pulses (i.e. a "burst") of laser light that provides sufficient energy to alter the exposed portion of photoresist so that its chemical composition differs from the chemical composition of the unexposed photoresist. The stepper shuts off the laser, repositions the wafer, and again the stepper fires the laser and exposes a portion of the photoresist to the laser light. The stepper continues this process of shutting off the laser, repositioning the wafer, and refiring the laser until the laser has exposed the entire layer of photoresist on the wafer. The stepper then replaces the wafer into the carousel, advances the carousel, and removes another wafer from the carousel. The stepper subsequently repeats the process of positioning the second wafer, firing a burst, and repositioning the second wafer until the photoresist on the surface of the second wafer is completely exposed. The stepper then repeats the process until all wafers in a carousel are exposed. The carousel is removed from the stepper, and another carousel is inserted to begin the process anew. A scanner operates similarly to a stepper, but the scanner includes scanning operations during which the beam is scanned across portions of the wafer. This is typically accomplished by moving the wafer and mask continuously under the beam.
Because of the stepping mode of operation in a stepper or scanner, the laser does not operate in a continuous or steady-state manner. The laser could be fired continuously or in a predictable periodic fashion, and the power of the beam could be used on demand by opening a shutter on the laser in order to have consistent operation of the laser. However, it is much more economical to stop firing the laser when the beam is not needed instead of firing the laser continuously and discarding the beam most of the time. The duty cycle for a laser used in conjunction with a stepper or scanner is typically between only about 10% and about 50%.
Intermittent operation of the laser creates transient phenomena that affect the consistency of the laser beam, and the transient conditions themselves vary substantially because of the varying "of" times associated with repositioning and realigning wafers, changing wafers, and changing carousels.